1. Field of the Invention
The present invention relates to a liquid crystal display (LCD) device, and more particularly, to a backlight unit including a lamp that does not use fluorescent material and an LCD device using the same.
2. Discussion of the Related Art
LCD devices, which are typically regarded as non-emissive display devices, have been widely utilized in notebook and desktop computers because of the advantage of having high resolution and displaying color images with high quality. An LCD device includes an LCD module that is provided with an LCD panel for displaying images and a backlight unit for supplying light to the LCD panel. In general, the backlight unit is classified as either side type or direct type, depending on the disposition of the light source.
For example, a fluorescent lamp may be utilized as a substantial light source for a side-type backlight unit. The side type backlight unit includes the fluorescent lamp, a light guide plate, a reflective sheet under the light guide plate, and a plurality of optical sheets. Examples of the fluorescent lamp include a cold cathode fluorescent lamp (CCFL), an external electrode fluorescent lamp (EEFL), and the like. Each of the CCFL and EEFL includes a glass tube, and first and second electrodes from both end portions of the glass tube. However, although the first and second electrodes in the CCFL face each other in the glass tube, the first and second electrodes in the EEFL cover the end surfaces of the glass tube.
FIG. 1 is a schematic cross-sectional view of a fluorescent lamp according to the related art. As shown in FIG. 1, a CCFL 29 includes a glass tube 11, and first and second electrodes 19a and 19b. The glass tube 11 is provided with a mixed gas 17 having an inert gas and mercury (Hg). The first and second electrodes 19a and 19b are disposed in the glass tube 11 to receive a high voltage through both end portions of the glass tube 11. For example, the first and second electrodes 19a and 19b may be shaped like a cup in the glass tube 11. Here, the glass tube 11 also includes a passivation layer 13 and a fluorescent material layer 15 that are sequentially formed on an inner surface of the glass tube 11.
Although not shown, there are first and second lead lines extending from the first and second electrodes 19a and 19b, respectively outside the glass tube 11. When the high voltage is applied to the first and second electrodes 19a and 19b through the first and second lead lines, an electron collides with the Hg molecule of the mixed gas 17 while the electron applied to one of the first and second electrodes 19a and 19b is moved into the other of the first and second electrodes 19a and 19b. Through the process, a secondary electron is radiated and the Hg molecule is excited. Therefore, an ultraviolet (UV) ray occurs while the excited Hg molecule is restored to a stable state. The UV ray is absorbed in the fluorescent material layer 15, and then the CCFL 29 generates a visible ray.
FIG. 2 is a schematic cross-sectional view of a side type backlight unit according to the related art. Although not shown, the CCFL 29 is arranged along a lengthwise direction of a light guide plate 23. As shown in FIG. 2, the CCFL 29 is arranged adjacent to a light receiving portion LP of the light guide plate 23. A lamp guide 33 surrounds a portion of the CCFL 29, which portion does not face to the light receiving portion LP. For example, the lamp guide 33 may be shaped like  as shown in FIG. 2. The light guide plate 23 has a top surface and a bottom surface. The bottom surface has a slope such that a thickness of the light guide plate 23 is reduced from the light receiving portion LP to an opposite portion to the light receiving portion LP of the light guide plate 23.
The related-art backlight unit also includes a reflective sheet 25 disposed under the light guide plate 23. For example, the reflective sheet 25 may be a white color or a silver color. Light “L” emitted from the CCFL 29 includes a first light L1 directly entering the light receiving portion LP,” and a second light L2 indirectly entering the light receiving portion LP after being reflected by the lamp guide 33 or the reflective sheet 25. The first and second lights L1 and L2 enter the light guide plate 23 through the light receiving portion LP and are diffused toward an entire surface of the light guide plate 23. And then, the first and second lights L1 and L2 are changed into a third light L3, which is a plan light passing through a plurality of optical sheets 21 on the light guide plate 23.
However, the brightness of the CCFL 29 is dependent on the flatness of the fluorescent material layer 15 (of FIG. 1). In other words, it is important to uniformly coat the fluorescent material layer 15 on the inner surface of the glass tube 11 (of FIG. 1). Thus, the process of coating the fluorescent material layer 15 becomes complicated in order to obtain the uniformly coated fluorescent material layer 15. Moreover, the brightness is reduced due to the lumping of the Hg molecule of the mixed gas 17 (of FIG. 1) and the fluorescent material layer 15. Moreover, the second light L2 of the CCFL 29 may be dissipated toward an opposite portion to the light receiving portion LP, and may partially enter the inside the CCFL 29 when it is reflected by the lamp guide 33. Thus, the second light L2 is absorbed in the fluorescent material layer 15, thereby causing a problem that the light for entering the light receiving portion LP is reduced.